I get this too! And they are hallucinations - hypnogogic hallucinations. It starts out for me in thought but then blossoms into something that feels very real.
Apparently it’s a normal phenomenon, I’ve noticed it’s more pronounced in those who struggle to get off to sleep.
Usually, when I try to explain it, it comes out as nonsense that makes complete sense to me. But once I’m awake, the sense is lost and it makes zero sense.
But it’s not always nonsense. Once I heard the most beautiful orchestra. The composition, the performance, it was truly beautiful. I can’t remember anything about it other than it was the most beautiful piece of music I’ve ever heard.
Coming in at a close second was an incredible guitar solo. It was a deep, emotional ballad expressed entirely on the fret board of an electric guitar. Again, truly beautiful. I look but I can never find anything that matches it. The closest I’ve come are some live performances of Buckethead.
It’s sad that I haven’t the ability to reproduce these experiences.
For the nonsense stuff, I’ve heard voices talking amongst themselves next to my bed. Sounds like they’re speaking English but what they say makes zero sense. Like grammatically correct sentences that mean nothing.
I’ve also watched pigs doing cartwheels while on a guided tour of a suburban neighbourhood.
I’ve never had the self-control to break it and note things down (by that point, I usually just want the sleep) but I have tried to apply ideas from behaviourism. And funnily enough it seemed to help.
On nights that are a particular struggle, I’ll let my mind wander and inject the odd bizarre, abstract thought, audio or visual in my minds eye. What I’ve noticed is that I seem to “catch the drift” and it triggers the whole hypnogogia thing that leads to sleep.
Some nights it doesn’t work but on those nights I’m far too tightly wound.